The present invention relates to semiconductor matrices which can be used in devices such as solar cells, diodes, electrifiers and transistors and the like and to methods of manufacturing such matrices from elemental semiconductors such as silicon and germanium.
An extremely important use of the semiconductor matrices of the present invention is in a solar cell. Because of the rapid depletion of our natural energy sources, and the associated pollution problems with these sources, solar energy as a source of energy has become highly important. Solar energy is abundantly available in this country and can be utilized without ecological problems.
Electricity can be directly generated from solar energy by the photovoltaic process whereby an electric current is generated when light is allowed to fall upon a rectifying contact or junction such as that contained in a solar cell. Photovoltaic solar cells are an expedient means of utilizing the virtually inexhaustable energy resource, incident solar radiation, to meet the growing worldwide demand for electric power.
However, it is well recognized that except in certain specific instances such as satellite space stations or remote installations away from the utility grid, photovoltaic electric generators are not cost effective at this time. This means that a cost penalty will be paid by an end user to employ power from a photovoltaic source compared to utility power derived from non-renewable fossile or nuclear fuels. For example, the costs of production of solar cells for use in satellites is approximately $20 to $50 per watt. This compares quite unfavorably with $0.50 per watt which is the approximately cost for fossile fuels.
A highly desirable objective is therefore to reduce the cost of power generated from solar radiation by the photovoltaic effect to the point where it becomes cost effective, thereby reducing consumption of and dependence upon non-renewable energy resources.
Reducton of cost can occur in any of several operations in the manufacture of photovoltaic solar cells. The disclosed invention focuses upon a method to reduce the cost of fabricating a semiconductor matrix.
An important factor in the cost of the fabrication of the semiconductor matrix is the manner of depositing the elemental semiconductor which is appropriately doped to form p-n junctions, on the substrate. Prior art methods expend and invest much energy in this process step which greatly increases the energy "payback" time of the resulting cells. That is to say, prior art techniques for manufacturing solar cells require the investment of so much energy that it takes several years to have this energy returned (payed back) as electricity from converted solar energy.
An example of this high energy investment in the deposition of silicon is found in Sirtl (U.S. Pat. No. 4,058,418) which discloses depositing silicon by introducing in a hydrogen stream silicon containing gases decomposable at temperatures at about 1,025.degree. C. and 1,200.degree. C. These high temperatures of course require significant energy investment. Carlson (U.S. Pat. No. 4,064,521) discloses a method of deposition as glow discharge which involves the discharge of electricity through gas at a relatively low pressure in a partially evacuated chamber.
Takagi (U.S. Pat. No. 4,066,527) discloses an ionized-cluster-deposition process which again requires significant expenditures of energy.
Janowiecki (U.S. Pat. No. 4,003,770) discloses producing a sprayed junction by plasma spraying a thin layer of silicon of opposite polarity or type over the initially deposited doped film.
Wakefield (U.S. Pat. No. 3,998,659) discloses forming a fluidized suspension of semiconductor particles which are maintained at a suitably elevated reaction temperature. A semiconductor containing vapor together with a dopant vapor and a vaporized reduction agent are then introduced into the fluidized suspension to effect deposition.
Other conventional techniques disclosed in Fang (U.S. Pat. No. 3,914,856) and Chu (U.S. Pat. No. 4,077,818) are epitaxial vaporation, electron beam evaporation, ion sputtering, thermal decomposition of silane, reduction of trichlorosilane or silicon tetrachloride with hydrogen at temperatures arranging from 900.degree. C. to 1,200.degree. C., or by thermal decomposition of dichlorosilane.
The Austin U.S. Pat. No. 3,990,953 discloses a method for electrodepositing elemental silicon on an electrically conductive cathode body. The patented method is an attempt to achieve a pure form of silicon which would subsequently be used in such applications as a solar cell. The electrodeposition occurs in an electroplating glass vessel with anodes and cathodes of platinum. A power supply is then used to drive the reaction to plate out elemental silicon on the cathode body.
The present invention discloses a method of producing semiconductor matrices by depositing the elemental semiconductor material while greatly reducing the cost and the energy investment required.